An arcjet is a simple, reliable form of electrothermal propulsion used to provide brief, low-power bursts of thrust, such a satellite needs for station-keeping. A nonflammable propellant is heated, typically changing state from liquid to gas, by an electric arc in a chamber. It then goes out the nozzle throat and is accelerated and expelled at reasonably high speed to create thrust. Arcjets can use electrical power from solar cells or batteries, and any of a variety propellants. Hydrazine is the most popular propellant, however, because it can also be used in a chemical engine on the same spacecraft to provide high thrust capability or to act as a backup to the arcjet.